"Line L has an undefined slope" so line L is any vertical line.
Anything perpendicular to this vertical line will be horizontal. So line M is horizontal.
Horizontal lines are of the form y = k for some fixed number k. The only thing that fits is choice B) y = 7 where k = 7 in this case.
Choice A graphs out a diagonal line, choice C graphs a vertical line, while choice D graphs out a hyperbola. These facts are why we can eliminate these non-answers.
Answer: Choice B) y = 7
Answer:
x=20
Step-by-step explanation:
The Pythagorean theorem is
a^2 +b^2 = c^2
where a and b are the legs and c is the hypotenuse.
The legs are x and 15 and the hypotenuse is 25
x^2 + 15^2 = 25^2
x^2 +225 = 625
Subtract 225 from each side
x^2 +225-225 = 625-225
x^2 = 400
Take the square root of each side
sqrt(x^2) = sqrt(400)
x = 20
Not necessarily.

and

may be linearly dependent, so that their span forms a subspace of

that does not contain every vector in

.
For example, we could have

and

. Any vector

of the form

, where

, is impossible to obtain as a linear combination of these

and

, since

unless

and

.